1. Field of the Invention
The present invention relates to both a chromatic dispersion compensation method and a polarization mode dispersion compensation method, which are essential to realize the current large capacity, high speed and long haul of an optical communication system. More particularly, the present invention relates to a system for optimally compensating for both chromatic dispersion and polarization mode dispersion in a super-high-speed single-wave or wavelength-division multiplex optical transmission system.
2. Description of the Related Art
With the recent rapid increase of network capacity, a far larger-capacity network is needed. Although currently a wavelength-division multiplex (WDM) optical transmission method based on the transmission capacity per channel of 10 Gb/s has been put into practical use, a far larger capacity will be needed in the future, and for the reasons of the use efficiency of a frequency and equipment cost, the realization of a high-speed optical transmission system with a capacity of 40 Gb/s per channel is expected.
However, since in such a super-high-speed optical transmission system, transmission waveform degradation due to both chromatic dispersion and polarization mode dispersion occurs, the transmission length of optical signals is restricted, which is a problem. Therefore, in order to realize such a super-high-speed optical transmission system, a system for compensating for both the chromatic dispersion and polarization mode dispersion with high accuracy is needed.
(I) About Chromatic Dispersion
Firstly, the summary of chromatic dispersion is described.
In an optical communication system with a transmission rate of 10 Gb/s or more, chromatic dispersion torelance remarkably decreases. For example, the chromatic dispersion tolerance of a 40 Gb/s NRZ (non-return-to-zero) signal is 100 ps/nm or less.
However, the repeater span of an optical communication system is not constant. Therefore, if 1.3 μm zero-dispersion single mode fiber (SMF) having a chromatic dispersion value of 17 ps/nm/km is used, the chromatic dispersion tolerance deviates if the repeater span differs only by several kilometers.
However, the distance of each repeater span and its chromatic dispersion value of an optical fiber transmission line possessed by a communication carrier are not accurately obtained, and it is often difficult to realize highly accurate chromatic dispersion compensation by a fixed chromatic dispersion compensation method using a DCF (dispersion compensation fiber) or the like.
Furthermore, since the chromatic dispersion value varies depending on fiber temperature, stress or the like as time elapses, it is necessary to optimally adjust the amount of chromatic dispersion of each repeater span by strictly measuring chromatic dispersion not only at the time of system operation commencement but also during the operation. For example, if the following conditions are assumed,
Type of optical fiber: DCF
Length of transmission line: 500 km
Temperature fluctuations: 100° C.
                                                                        [                                  Amount                  ⁢                                                                          ⁢                  of                                                                                                                          chromatic                  ⁢                                                                          ⁢                  dispersion                                ]                                                    =                ⁢                  [                                    Temperature              ⁢                                                          ⁢              dependency              ⁢                                                          ⁢              of              ⁢                                                          ⁢              zero                        -                                                                      ⁢                      dispersion            ⁢                                                  ⁢            wavelength                    ]                ×                                        ⁢                              [                          Amount              ⁢                                                          ⁢              of              ⁢                                                          ⁢              temperature              ⁢                                                          ⁢              change                        ]                    ×                                                ⁢                              [                          Dispersion              ⁢                                                          ⁢              slope              ⁢                                                          ⁢              of              ⁢                                                          ⁢              transmission              ⁢                                                          ⁢              line                        ]                    ×                                                ⁢                  [                      Transmission            ⁢                                                  ⁢            distance                    ]                                        =                ⁢                  0.03          ⁢                                          ⁢                      nm            /            °                    ⁢                                          ⁢                      C            .                    ×          100          ⁢          °          ⁢                                          ⁢                      C            .                    ×                                                ⁢                  0.07          ⁢                                          ⁢                                    ps              /                              nm                2                                      /            km                    ×          500          ⁢                                          ⁢          km                                        =                ⁢                  105          ⁢                                          ⁢                      ps            /            nm                              
This value is almost equal to that of the chromatic dispersion tolerance of a 40 Gb/s NRZ signal. Therefore, an automatic chromatic dispersion compensation system for optimally controlling the amount of chromatic dispersion compensation is essential to not only a system using an SMF as a transmission line but also one using 1.55 zero-dispersion shift fiber (DSF) and NZ (non-zero)-DSF as a transmission line.
(II) About Polarization Mode Dispersion
Next, polarization mode dispersion (PMD) is described.
PMD is caused by a difference in propagation delay time between the polarization components (such as between a TE mode and a TM mode) of an optical signal, and is caused in an optical fiber.
Generally, the larger an optical signal is, the larger the influence of polarization mode dispersion becomes. The longer the transmission distance is, the larger the influence of polarization mode dispersion becomes. The influence cannot be neglected.
There is one with a large PMD value per unit length of 1 ps/km1/2 (pico-second/km1/2) (pico indicates 10−12) in optical fibers constituting an old optical transmission line mainly laid outside Japan. Even if a short-haul transmission (such as 50 km transmission) is conducted using such an optical fiber, optical differential delay (Δτ) is 7 ps or more against one time-slot 25 ps of a 40 Gb/s NRZ signal. Therefore, the influence of polarization mode dispersion cannot be neglected like the earlier-mentioned chromatic dispersion. In reality, since in an optical communication system, materials that cause polarization mode dispersion, such as an optical amplifier, a chromatic dispersion compensator and the like must be provided in a transmission line, there is a possibility that the transmission length of optical signals is further restricted. Furthermore, since polarization mode dispersion varies depending on stress applied to an optical fiber and temperature change as time elapses, the state of polarization mode dispersion in a transmission line must be monitored not only at the time of installation but also during its operation, and the polarization mode dispersionmust be dynamically compensated.
As described above, chromatic dispersion and polarization mode dispersion are major factors in limiting the performance of an optical communication system. Therefore, in order to improve the performance of the optical communication system, an automatic dispersion compensation system for independently and dynamically compensating for both chromatic dispersion and polarization mode dispersion must be prepared.
Device technologies for realizing the automatic dispersion compensator are grouped into the following three items (a) through (c).    (a) Realization of a variable chromatic dispersion compensator    (b) Realization of a transmission line dispersion monitor    (c) Realization of the feedback optimization control method of a variable chromatic dispersion compensator
As a chromatic dispersion compensator in (a) above, the following ones are proposed as examples.
(1) VIPA (Virtually Imaged Phased Array)
“Variable Dispersion Compensator Using the Virtually Imaged Phased Array (VIPA) for 40 Gbit/s WDM Transmission Systems”, ECOC2000, PD Topic 2, 2.3
(2) Tunable Ring Resonator
“Tunable Ring Resonator Dispersion Compensators Realized in High Refractive-index Contrast Technology” ECOC2000, PD topic 2, 2.2
(3) FBG (Fiber Bragg Grating)
“Twin Fiber Grating-Adjustable Dispersion Compensator for 40 Gbit/s” ECOC2000, PD Topic 2, 2.4
As the polarization mode dispersion compensator, the following ones are proposed as examples.    (1) A method for providing a polarization controller (PC) at an optical signal transmitting terminal, feeding back its transmission characteristic from its receiving terminal and controlling the branch ratio γ of its optical intensity to two polarization modes so as to be 0 or 1.
“Optical Equalization of Polarization Dispersion”, SPIE Vol. 1. 1787 Multi-gigabit Fiber Communications, 1992, pp. 346-357    (2) A method for providing both a polarization controller and a polarization maintaining fiber (PMF) at an optical signal receiving terminal and giving a differential delay between two polarization modes with a sign the reversal of that of an optical transmission line by controlling the polarization controller.
“Automatic Compensation Technique for Timewise Fluctuating Polarization Mode Dispersion in In-line Amplifier Systems”, Electro. Lett., Vol. 30, No. 4, 1994, pp. 348-349    (3) A method for providing a polarization controller, a polarization beam splitter, two light receivers receiving each of two demultiplexed optical signal components and a variable delay device giving a differential delay between two electrical signals obtained by these light receivers, and controlling both the polarization controller and variable delay device.
“Polarization Control Method for Suppressing Polarization Mode Dispersion Influence in Optical Transmission Systems”, Journal of Ligthwave Technology, Vo. 12, No. 5, 1994, pp. 891-898
Next, as to the transmission line dispersion value monitor essential to the feedback/control in (b) above, several methods are proposed.
Firstly, as the measurement method of chromatic dispersion values, a pulse method and a phase method for inputting a plurality of segments of light each with a different wavelength and a group delay or a phase difference between a plurality of segments of output light has been conventionally proposed. However, in order to always conduct chromatic dispersion measurements without degrading communication quality during system operation, using these methods, (1) one set of chromatic dispersion measurement instruments are needed for each repeater span, and (2) wavelength-division multiplexing must be applied to a plurality of segments of measurement light each with a wavelength different from that of a data signal, which are problems. However, it is not practical from the viewpoints of economy and equipment size to realize these.
Several methods are proposed on a chromatic dispersion monitor used to solve such problems. Examples of such a chromatic dispersion monitoring methods are as follows.
(1) A Method Utilizing the Principal that the Intensity of a Specific Frequency Component Varies Due to Waveform Distortion and Using the Intensity of a Specific Frequency Component of a Received Baseband Signal
(“Automatic Dispersion Equalization in 40 Gbit/s Transmission by Seamless Switching between Multiple Signal Wavelengths”, ECOC′ 99, pp. 1-150-151)
(2) A Method Using an Error Rate
In this method, an error rate is monitored by a receiver, and feedback/control is exercised over a chromatic dispersion compensator so that the error rate may become the best.
In any of practical dispersion monitors, waveform distortion due to dispersion is directly or indirectly used. If chromatic dispersion and polarization mode dispersion are simultaneously caused, it cannot be determined by which waveform distortion is caused, chromatic dispersion or polarization mode dispersion. Therefore, in this case, it is difficult to realize an automatic dispersion compensator for simultaneously compensating for both chromatic dispersion and polarization mode dispersion.
As the polarization mode dispersion measurement method, the following ones are proposed.    (1) Senarmont Method    (2) Rotary analyzer method    (3) Rotary phase-retarder method    (4) Phase Modulation method
As the polarization state indication (expression) method, the following ones are proposed (“Indication Method and Measurement Method of Polarization State”, OPTRONICS, No. 5 pp. 109-117 (1997)).    (1) Poincare sphere    (2) Jones vector    (3) Stokes vector
As an example, a polarization mode dispersion measurement method using Jones vector and a device thereof is proposed by Japanese Patent Laid-open Application No. 9-72827. Although it is difficult to apply it in an environment where chromatic dispersion exists, a polarization mode dispersion monitor monitoring a specific frequency component in a received signal is also proposed (this applicant is now filing it for a patent).